The proposed research intends to evaluate the functional significance of the surface glycoproteins of hemopoietic stem cells (CFC) which produce macroscopically visible spleen colonies when transplanted into irradiated hosts. Surface glycoproteins will be perturbed by shearing sialic acid with neuraminidase and surface glycoproteins more generally with proteases. Enzyme effects of stem-cell capability will be measured (1) in terms of colony number per uniform cell number injected, and (2) assuming that the cells are affected, their ability to repair surface sites in an in vitro system already demonstrated to allow stem cell renewal after eclipse. Attempts will be made to evaluate both the biochemical nature of the sheared material and the types of surface complexes produced during the repair process. Isotopic analysis will be used to investigate early proliferation kinetics and differentiative capability of cells which remain or have repaired sufficiently to regain CFC ability. The information gained from this study will be employed in development of techniques fundamental to the biology of blood cells; the purification of specific stem cells from parent tissue and the enrichment and persistence of stem cells in culture systems. The rationale for these studies is that the cell surface is a fundamental cell characteristic and can be exploited in the form of affinity separation columns for preparation of stem cell populations. The single fact which characterizes stem cells is a lengthened or arrested cell cycle. The author believes that cells are maintained in this state by surface inhibition of proliferation and differentiation. A systematic survey of culture conditions which provide optimal conditions for maintenance of the Go phase is proposed. The successful development of these techniques will provide cells for clinical transplantation as well as for future studies of therapeutic agents and their utilization in the control of neoplastic growth.